A planetary gear transmission which comprises a combination of two planetary gear trains such as Ravigneaux gear train and Simpson gear train, has been well known in the prior art. Although this type of planetary gear transmission is generally designed with four forward gear positions or four speed ranges, some transmissions are constructed with five gear positions since there is demand for more speed ranges to improve vehicular traveling performance.
As such a transmission which provides an additional speed range, a planetary gear transmission comprising three planetary gear trains is disclosed, for example, in Japanese Laid- Open Patent Publication No. 59(1984)-222644 or in Japanese Laid- Open Patent Publication No. 1(1989)-320361. In this type of transmission, the planetary gear trains are combined with three or four clutches and three brakes (these clutches and brakes are referred to as "engaging means") so that, by controlling the actuation of the clutches and brakes in combination, the planetary gear transmission is made capable of providing five forward gear positions and one reverse gear position. However, this type of transmission has a few disadvantages: the size and construction of the transmission is relatively large and complex as it requires a number of engaging elements (clutches and brakes) for establishing the respective speed ranges; the efficiency of power transmission is often reduced by the rotational resistance of the engaging elements used; and the pinion gears (or the gears which are supported on the carriers) of the planetary gears may experience over- rotation.
Therefore, the applicant of the present invention has proposed a transmission which is disclosed in Japanese Laid- Open Patent Publication No. 4(1992)-125345. This transmission comprises three coaxially disposed planetary gear trains and five engaging means (three clutches and two brakes) to achieve a relatively small size, to improve the power transmission efficiency, and to prevent the pinion gears from over- rotation.
However, in this transmission, the practical range of gear ratio for the components (a sun gear, a carrier, pinion gears and a ring gear) of each planetary gear is substantially limited by the relatively small size of the respective planetary gear. In addition, the gear ratio (or speed ratio) for each speed range is limited within a certain range because a change made to the gear ratio of any one of the planetary gear trains can affect the gear ratios of all the speed ranges. Thus, there is a problem that the degree of freedom in determining the gear ratios are relatively small.